1. Field of the Invention
The present invention relates to a technique of controlling the charging process in a secondary battery and a power output apparatus that utilizes the control technique. More specifically, the present invention pertains to a technique of controlling the charging process in the secondary battery, which is subjected to repeated charge and discharge in a specific charged state that is lower than a full charge level, and thereby preventing the secondary battery from being charged excessively.
2. Description of the Related Art
Typical examples of the secondary battery include lead acid batteries, nickel-cadmium batteries, nickel-hydrogen batteries, and lithium batteries. After consumption of the electric power, these batteries are connected to an external power source and charged with a certain supply of electric current fed from the external power source. The secondary batteries have been used for a variety of apparatuses by taking advantage of these characteristics. By way of example, the secondary battery is mounted on the conventional vehicle to supply the electric power to an ignition plug of the engine. The secondary battery is also used as a main power source for driving the motor in a hybrid vehicle with the engine and the motor mounted thereon.
Each battery has a preset limit in chargeable amount of electric power. It is accordingly required to regulate the charging amount of the battery within the range of its chargeable capacity. If the charging amount of the battery exceeds this limit, that is, if the battery is excessively charged, the life of the battery may undesirably be shortened.
The technique of preventing the battery from being charged excessively is disclosed in, for example, JAPANESE PATENT LAID-OPEN GAZETTE No. 8-298140. This technique detects a temperature variation of the battery in the course of charging and determines that the charged state of the battery reaches the full charge level when the temperature variation per unit time, that is, the temperature gradient, abruptly increases. FIG. 13 is a graph showing time-based variations in electric current, voltage, and temperature of the battery in the course of charging. When a fixed amount of electric current I1 is supplied to charge the battery as shown in FIG. 13(a), the temperature of the battery abruptly increases in the charged state close to the full charge level. This state is shown by a curve Temp1 in the graph of FIG. 13(c). At a time point t1 in FIG. 13(c), the charged state becomes close to the full charge level. The temperature gradient in this charged state corresponds to the slope of a tangent at each time point in the curve Temp1. For example, the temperature gradient at the time point t1 is expressed as a straight line m1 in FIG. 13(c), and is significantly greater than the temperature gradient at another time point t2. The conventional technique takes advantage of this characteristic of the battery to detect the full charge state of the battery and stops further charging, thereby preventing the battery from being charged excessively.
In the conventional battery, once the charging process starts, the battery is continuously being charged until the charged state reaches the full charge level. This means that charging of the battery is not carried out under the circumstances where the charge and discharge are frequently repeated in a specific charged state that is lower than the full charge level. In this case, the temperature of the battery monotonically increases as shown in FIG. 13(c). The full charge state of the battery is thus determined in response to an abrupt increase in temperature gradient.
The battery mounted on, for example, a hybrid vehicle, however, may be charged and discharged in a repeated manner during a run of the vehicle. In the hybrid vehicle, in the case where the output of the engine is greater than the required power for driving, the surplus power is used to drive the generator and charge the battery. In the case where the output of the engine is smaller than the required power, on the other hand, the electric power is discharged from the battery to drive the motor and supplement the insufficiency of the power. Such repeated charge and discharge is carried out according to the driving state of the vehicle, the charged state of the battery, and the instruction of the driver.
The battery generally evolves the Joule heat accompanied with the chemical reaction in the course of discharging. When the charge and discharge are repeatedly carried out, for example, like in the case where the battery is mounted on the hybrid vehicle, the temperature of the battery does not monotonically increases as shown in FIG. 13(c). FIG. 14 is a graph showing time-based variations in electric current, voltage, and temperature of the battery in the case of repeated charge and discharge. For example, if discharge occurs in the course of charging the battery as shown in FIGS. 14(a) and 14(b), the temperature of the battery is increased by the Joule heat during the discharge and has a variation as shown in FIG. 14(c).
When the temperature of the battery varies in this manner, the temperature gradient at a time point t2 (the slope of a tangent m2) may become identical with the temperature gradient at a time point t1 (the slope of a tangent m1) in FIG. 14(c). At the time point t2, however, the charged state of the battery is not close to the full charge level. The control procedure that determines the full charge state of the battery based on the temperature gradient mistakenly determines that the charged state of the battery reaches the full charge level at the time point t2 and stops the charging process. This arrangement prevents the battery from being sufficiently charged and may cause the battery to untimely die or have other problems.
One available structure determines whether or not the charged state of the battery is close to the full charge level with a sensor for detecting the charged state of the battery. Another available structure integrates the electric currents fed into and discharged from the battery and thereby determines whether or not the charged state of the battery is close to the full charge level. In the former structure, however, there is a fear that the sensor for detecting the charged state of the battery malfunctions. In the latter structure, on the other hand, the error in integration may lead to wrong determination of the full charge state. It is accordingly desirable to independently determine whether or not the charged state of the battery is close to the full charge level, even if one of these methods is adopted in detection of the charged state of the battery.